Conventional auto-ranging instruments are provided with an internal range switching unit to enable automatic switching of a measured signal so that it can be accommodated within the scale ranges provided on the analogue to digital conversion unit (hereinafter referred to as A/D converter).
Currently available devices employ different techniques for auto-ranging; for example, a U.S. Pat. No. 4,827,191, teaches a technique of measuring the peak values and entering them into the control section of an A/D converter to select an appropriate scale range of a measuring instrument. Another U.S. Pat. No. 3,813,609, discloses a technique of adjusting the level of the measured signal based on the output power of an A/D converter.
However, the technique according to U.S. Pat. No. 4,827,191 is applicable to A/D converters equipped with range-switching capabilities, but not to those without such capability. On the other hand, U.S. Pat. No. 3,813,609 teaches that the time between the reception of a signal and its digital output is primarily governed by the processing speed of A/D conversion. Precision measurements require the use of an A/D converter having a large number of bits, resulting in relatively slow response. In order to shorten the response time, converters such as successive approximation or flash type is required, both of which are relatively expensive, thus making the instruments containing such devices more costly.
The operation of such an A/D conversion unit similar to the one described in the above mentioned U.S. Pat. No. 3,813,609 is described in the following.
FIG. 3 shows a schematic diagram of an instrument having the A/D converter mentioned in the latter patent above. In this instrument, photoelectric sensor component 1, e.g. photodiode, receives light energy and outputs it as electrical energy. The range-switching amplifier 2 for the range-switching unit receives a signal from said component 1 and amplifies the signal according to a preset amplification factor. The range-switching amplifier 2 is set to change the amplification factor according to the magnitude of the supplied signal S.sub.c. The measuring A/D converter 3 converts the signal from the range-switching amplifier 2 into a digital form. The microprocessor 4 consists essentially of a CPU, a ROM, a RAM and interface circuits (none of which is shown). A control program for the CPU is stored in the ROM, which also contains data regarding the input range of the measuring A/D converter 3, i.e. the scale range limits.
Microprocessor 4 receives the signal from the A/D converter 3, converts it into numerals and forwards the result to a display unit 5. Microprocessor 4 also generates a control signal to vary the amplification factor of the range-switching amplifier 2 so that its output signal to the A/D converter 3 will be within the input scale range. The microprocessor 4 and the range-switching amplifier 2 constitute the range-switching circuitry.
When the magnitude of the output signal from the amplifier 2 into the A/D converter exceeds the scale range of such a light intensity measuring device, the excess signal overflows from the converter. When the microprocessor 4 receives such an overflow indication, it generates a command signal to decrement the amplification factor of the range-switching amplifier 2. The amplifier 2 changes the amplification factor accordingly by a certain amount. If the microprocessor 4 still senses the overflow signal from the amplifier 2, it outputs another command signal to lower the amplification factor still further. This process is repeated until the cessation of the overflow signal, from the amplifier 2 into the microprocessor 4, indicating that the magnitude of the incoming signal is within the scale range of scale of the A/D converter 3. At this time the microprocessor 4 accepts the data from the converter 3, transforms the data into a numerical form and displays the result on the display unit 5. On the other hand, when a low level signal is received, the microprocessor 4 continues to increase the amplification factor until the signal magnitude reaches a measurable scale range.
As described above, the conventional meters operate by following incremental trial processes of changing the amplification factor of the range-switching amplifier 2 until the data fit into one of the scale ranges of the measuring device.
Such instruments have the following problems. First, the response time of such circuits is necessarily long because the whole process is predicated on successive trials of changing the amplification factor until the results fit into a permissible range. For example, if an integrating-type A/D converter is used, the conversion times can range from several milliseconds to several hundreds of milliseconds, making the total response time well over one second for five trials. If a faster successive approximation-type converter is used, the conversion time could be shortened to several milliseconds. However, such converters are relatively more expensive on the basis of the number of bits.